(1) Field of the Invention
The present invention relates to a switching power supply of the voltage resonance type which is used as an inverting switching power supply of a microwave oven.
(2) Description of the Prior Art
In order to obtain a large amount of power output in a small size, a switching power supply of voltage resonance type has been used. Such a power supply is illustrated in FIG. 7 and is formed by adapting a parallel resonance of a resonance capacitor (1) and an inductance (L) of a leakage transformer (2) which functions as an inverter transformer. Such a system is used even though it only has an efficiency of about 50% of a magnetron. As is known, such a voltage resonance type switching power supply fixes an OFF period relative to an input voltage (Vin) based on the value of the resonance frequency ##EQU1## After the resonance voltage which is applied on a switching element (3) is lowered to OV, switching element (3) is energized. Therefore, it is possible to obtain an improved power circuit having a small amount of switching loss and a high conversion efficiency.
However, as mentioned above, since the OFF period is fixed by the resonance frequency and the output is stabilized by changing the ON period in the voltage resonance type switching power supply, it is necessary to make large changes in the switching frequency. Therefore, a PWM (Pulse Width Modulation) system which was used for the conventional switching power supply cannot be used. Accordingly, a conventional voltage resonance type switching power supply has to contain extremely complicated control circuit. Namely, circuits are required for turning the switching element (3) to ON or OFF state relative to a reference signal. Circuits which are required for the externally excited type of power supply are, as shown in FIG. 7, a rectifying and smoothing circuit (4) for the input power, the switching element (3), the inverter transformer (2), a magnetron (5), voltage detecting circuit (6) for detecting whether the voltage of switching element (3) is lowered to OV, a circuit (7) for setting the ON period of the switching element (3), a V/F converter (8) for adjusting switching frequency which forms a reference at high speed to obtain the above noted ON period, a power detecting circuit (9) for detecting magnetron power, and a circuit (11) comprising essentially a pulse transformer for insulating the control signal fed to a control circuit (10). Since the microwave oven is an article used by many people and it is required to satisfy the contradictory conditions of high reliability and low cost, it is necessary to realize the oven using simple circuits as much as possible. Since the conventional inverter circuit for microwave oven used the voltage resonance type switching power supply, the control circuit had to be complicated. Further, since a small signal was defined as a reference signal, it was necessary to protect against an error caused by external noise. Therefore, peripheral circuits were required to be added as protective circuits, thereby causing a limitation in the criteria of high reliability and low cost.
The object of the present invention is to provide an improved switching power supply for a microwave oven having high reliability and low cost without any usage of complicated control circuit.
Other objects of the present invention will be clarified by the following explanation.